A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. A typical RAT in current use is described in U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power, or both in the case of a hybrid RAT.
As described in Markunas et al., the turbine includes a speed governor that changes the turbine blade position to maintain a relatively constant shaft speed to the power generating equipment. Failure of the turbine speed governor can cause an over-speed condition that may ultimately cause the release of a turbine blade at very high speed. Due to the high speed, the wayward blade has very high energy as well. The most common cause of governor failure is due to seizure of the bearing between the turbine shaft and the governor yoke plate that controls the pitch of the turbine blades.
The potential release of a high-energy blade proximate the aircraft is a concern for both commercial and military RAT applications. Should the wayward blade strike the aircraft fuselage, it may penetrate the skin and cause damage to electric or hydraulic power equipment or control systems. It may also injure passengers or crew. If one of the propulsion engines ingests the wayward blade, the engine may suffer severe damage that results in loss of thrust.
Current methods to minimise hazards caused by turbine over-speed-induced release of a turbine blade have involved strategic placement of key elements or shields to prevent penetration. These methods no longer satisfy increasingly stringent certification and safety requirements promoted by airworthiness authorities.